The present invention relates to a die-casting method for production of die-cast products useful not only as structural members but also as functional members and die-cast products manufactured thereby.
In a conventional die-casting method, molten aluminum or aluminum alloy (hereinafter referred to as "molten metal") poured into a sleeve is forcibly injected into a cavity of a die-casting mold by a plunger. Most of gases such as air and water vapor are purged from the cavity in response to injection of the molten metal, but some of the gases remain as such in the cavity even after the injection. Especially, die-casting molds designed for production of thin-walled products or products having complicated configurations have portions acting as bottlenecks against gas flow, so that it is difficult to completely remove gases from the cavity.
Gases trapped in the cavity are included in a cast product, when the injected molten metal is cooled and solidified in the cavity. Inclusion of gases causes defects such as blowholes and porosity in die-cast products. Therefore, the die-cast products obtained in this way have been regarded as members unsuitable for functional uses, e.g. scrolls, pistons, cylinder blocks, connecting rods or suspension parts, due to poor mechanical properties. If cast defects derived from inclusion of gases are suppressed, a die-casting method excellent in productivity can be applied to various fields of technology.
In order to eliminate harmful influences derived from inclusion of gases, a vacuum die-casting method was proposed. According to the vacuum die-casting method, a cavity of a die-casting mold is evacuated before injection of molten metal, so as to remove gases from the cavity. The cavity is held at a degree of vacuum in the range of 200-500 millibar by evacuation. However, an internal pressure of the cavity can not be reduced less than said value, due to invasion of air through narrow gaps of dies. Invasion of air also occurs during the pouring of molten metal into a sleeve. As a result, cast defects such as porosity caused by inclusion of gases are detected even in products obtained by the vacuum die-casting method, although inclusion of gases is somewhat decreased as compared with products obtained by a conventional die-casting method. In this regard, the products are not good enough for use as functional members.
An oxygen die-casting method has been developed in order to eliminate defects in the vacuum die-casting method. According to the oxygen die casting method, as disclosed in JP B 50-21143, a cavity of a die-casting mold is filled with oxygen at a pressure higher than the atmospheric pressure so as to replace gases by oxygen prior to injection of molten metal. Since oxygen gas fed into the cavity is effused through narrow gaps of dies as well as an injection hole, invasion of atmospheric gas through the narrow gaps or the injection hole can be prohibited. In addition, the oxygen gas fed into the cavity is reacted with molten metal, and a reaction product Al.sub.2 O.sub.3 is dispersed as fine particles in a cast product without harmful influences on an obtained die-cast product.
However, complete replacement of gases from the cavity of a die-casting mold by oxygen injection is substantially impossible, even when oxygen is fed into the cavity at a pressure higher than the atmospheric pressure. Gases often remain at difficult portions for the replacement in the cavity. A die-casting mold designed for production of a product having a complicated configuration has difficult portions to which oxygen is hardly reached, so that gases such as air and water vapor can not be replaced by the fed oxygen but remain as such. The residual gases and water vapor from parting agents are included in products and cause defects.
Residual air can be efficiently removed from the cavity by oxygen blowing during evacuation, as disclosed in JP B 57-140. However, simultaneous oxygen blowing with evacuation is not effective for removal of water vapor. In fact, cast defects caused by inclusion of gases are still detected in a cast product obtained by this method. JP B 1-46224 discloses another die-casting method, wherein oxygen blowing is performed after evacuation. However, some cast defects are also detected in a cast product, since a cavity of a die-casting mold is held at a decompressed pressure during the oxygen blowing.
Inclusion of the trapped gases also causes blisters in die-cast products, when the die-cast products are heat-treated in such as T6 treatment (i.e., solution heating, quenching and then aging) for improvement of mechanical properties. In order to avoid such blisters, most of die-cast products are not used with heat treatment.